1. Field of the Invention
This invention relates generally to carbon composite products and the methods of making same. More particularly, the invention relates to a novel product manufactured by a process wherein a porous, rigidized shaped substrate, or preform, is formed from selected carbonized or graphitized fibrous materials which are bonded together under controlled conditions of temperature and pressure by small amounts of a carbonaceous binder. To complete the composite product, the substrate is controllably densified by the chemical vapor deposition of pyrolytic carbon interstitially thereof.
2. Discussion of the Prior Art
Excellent high temperature performance characteristics of carbon composites in structural, frictional, ablative and thermal insulation applications has caused an ever expanding demand for such materials. Accordingly considerable effort has been expended in recent years in developing new techniques for the large scale production of such materials for use in nuclear, aerospace, aircraft and industrial fields.
In producing carbon composite products, substrates or preforms, made up of carbon fibrous materials are first constructed. Two basic methods are typically employed to produce the substrates. In accordance with one method, sometimes called the resin bonding method, the fibrous substrate materials are bonded together using substantial amounts of carbonizable binder such a phenolic resin or the like. The methods described in the patent to Bickerdike et al, U.S. Pat. No. 3,233,014 is exemplary of the resin bonding method. In accordance with the second method, often called the Chemical Vapor Deposition or C.V.D. method, the fibrous substrate materials are bonded together by the interstitial deposition of pyrolytic carbon using known chemical vapor deposition (C.V.D.) techniques. The patent to Bauer, U.S. Pat. No. 3,895,084 clearly describes the C.V.D. method. Also pertinent to this method is the British Pat. No. to Williams, 1,455,891.
The resin, or carbonaceous binder method has certain recognized advantages. For example, using this method, durable, high density structurally stable preforms can readily be produced using conventional molding and press bonding techniques. The preforms formed by this method can conveniently be shaped and are easily handleable. Drawbacks of the resin binder method, however, include the fact that the method is relatively expensive requiring several preparatory processing steps including pre-impregnation of the substrate materials, "B" staging and binder carbonization. Additionally, difficulties have been experienced using this method in achieving acceptable compatability between fiber and binder processing shrinkages, and in routinely producing preforms which retain acceptable part integrity during the various substrate processing steps without delaminating or microcracking. In a similar vein, the relatively high quantities of carbonizable binder necessary to produce an acceptable carbonizable substrate frequently pose debulking, outgassing and dimensional stability problems during carbonization of the binder. A frequent objective of the resin bonding method is to totally encapsulate the individual fibers of the substrate in an attempt to eliminate undesirable voids in the substrate.
The C.V.D. method, wherein substrate bonding is accomplished by depositing onto the fibers of the substrate carbon resulting from dissociating methane or other carbon bearing source gasses, also has several recognized advantages. For example, this technique produces a substrate having maximum open porosity so as to permit precisely controllable partial or complete substrate densification. Additionally, inherent in the method is the fact that each fiber of the substrate is uniformly coated with the deposited material rendering it substantially impermeable and unusually resistant to corrosion even at high temperatures. Further, the thickness of the coating on the fibers themselves and at the fiber crossover points can be precisely regulated so as to achieve the desired substrate rigidity.
A major disadvantage of the C.V.D. method is that some form of expensive and often bulky shaping fixture is required to hold the substrate materials in the desired configuration until sufficient pyrolytic carbon has been deposited to rigidize the fibrous structure. Such hardware is expensive, reduces furnace productivity substantially since it occupies a significant portion of the severely limited furnace processing volume, and presents significant assembly and disassembly problems.
As will become apparent from the discussion which follows, the method of the present invention permits realization of the advantages of both the resin and C.V.D. methods while substantially avoiding the disadvantages of each method.